The present invention relates to a method of making activated carbon and, in particular, a process that can be performed rapidly in an open atmosphere with reduced or no tar formation.
Activated carbon or activated charcoal is a versatile material having very high specific surface area arising from its structurally porous form of carbon. Based on the dimension of the pores, they are classified as micro, meso and macro porous carbon respectively with characteristic pore dimensions around <2 nm, 2-50 nm, and >50 nm, according to IUPAC standards. The specific surface area of activated carbon can vary from 200 M2/g to 2000 M2/g. Activated carbons are widely used in purification of water and other liquids, air purification, solvent recovery, chemical catalysis, electrochemical device components and other wide variety of industrial processing.
Activated carbon is generally prepared by pyrolysis of carbon precursor material, by heating material to a temperature greater than 500° Celsius in an oxygen free environment. This enables removal of volatile materials and liquid pitch leaving behind char, a carbon rich material. This carbonized product is then activated at temperatures above 800° Celsius under steam or CO2 to promote reactions that enable pore formation. Carbon precursor materials are various plant products generally described as biomass.
Control over porosity of activated carbon can be obtained by using a chemical activation agent such as zinc chloride, potassium hydroxide, or sodium hydroxide mixed with carbonized product, usually less than 10% by weight of carbon precursors. These chemical agents act as a template for the creation of pore structure during activation. In some preparation routes, the chemical reagent is added to the uncarbonized precursor material and a pyrolysis process is carried out to prepare a carbonized product and then the material is activated. In either of these processes, elimination of the reagents by additional chemical reactions and subsequent washing is required to obtain activated carbon.